The present invention relates to a system for examining subsurface environments, and more particularly, to a method for detecting visual images of subsurface geological environments using a soil penetrating probe.
Increasing concern with soil and groundwater contamination and governmental mandated requirements to clean up hazardous waste sites have created a need for cost effective systems and methods for determining the characterization of subsurface environments. In response to such needs, soil penetrating probes have been developed. Soil penetrating probes generally comprise a tube having a tapered tip which is forced down into the ground. Instrumentation in the tube detects various properties of the surrounding geological environment.
U.S. Pat. No. 5,128,882, xe2x80x9cDevice for Measuring Reflectance and Fluorescence of In-situ Soil,xe2x80x9d describes a soil penetrating probe having an optical fiber, a light source within the interior of the probe, and a transparent window which provides a light port into and out of the probe. Light passes through the transparent window to irradiate the surrounding soil immediately outside of the window as the probe passes through the soil. The irradiated soil reflects light back through the window whereupon the reflected light is collected by a fiber optic link connected to instrumentation on the surface. The collected light then is subjected to spectroanalysis for determining the chemical composition of the soil, particularly with regard to soil contamination. This system only detects the spectral characteristics of the surrounding environment; it cannot provide optical images. Therefore, information such as the porosity and grain size of surrounding soils are not discernible from the type of information provided through spectral analysis. However, porosity and grain size are important characteristics because they are important variables that control the transport of contaminants in soil.
Another soil penetrating probe system is described in U.S. Pat. No. 5,123,492, xe2x80x9cMethod and Apparatus for Inspecting Subsurface Environments.xe2x80x9d This system includes a soil penetrating probe having a clear tube in which is suspended a video camera linked to the surface. A significant limitation of this system is that because the camera freely swings within the transparent tube, the focus of the camera with respect to the surrounding geological features is constantly changing and cannot be controlled. Furthermore, the system does not provide any means for illuminating the surrounding subsurface environment other than from ambient light which may happen to filter from the surface down through the tube.
Delineation of Non-Aqueous Phase Liquids (NAPLs) contaminants in the subsurface soil environment is a serious environmental challenge. In particular, locating Dense Non-Aqueous Phase Liquids (DNAPLs) is recognized as one of the most difficult technical challenges currently limiting the clean up of hazardous wastes sites. Much of the problem is because DNAPLs behave differently than most other contaminants. Because they are immiscible with, and denser than, water, they do not accumulate on the surface of ground water, as is usually the case with petroleum hydrocarbon contaminants. Instead, when they are released into the environment as a result of surface spills, tank leaks, and improper disposal practices, they tend to sink through the vadose zone, through the capillary fringe and on into the ground water. As they sink, they can leave behind a trail of micro-globules in the pore spaces of the soil matrix. Because they have relatively high volatilities, residual phase chlorinated solvents in the vadose zone can be transferred quickly into the vapor phase and out of the soil. Consequently, vadose zone contamination is generally not the most significant long-term problem. In contrast, when chlorinated DNAPLs make it into the groundwater, transfer into the vapor phase no longer remains an effective removal mechanism. Instead, the residual immiscible xe2x80x9cfree-productxe2x80x9d phase can slowly dissolve into the groundwater any where from decades to centuries. Even though the solubilities are low enough to allow the residual phase to persist for very long times, the solubilities are high enough to result in water concentrations many orders of magnitude greater than current drinking water quality standards.
The fact that the source of the DNAPL contamination to the groundwater often exists as tiny xe2x80x9cmicro-globsxe2x80x9d of residual free product trapped between individual soil particles is one of the main reasons that site remediation has been so difficult. To make matters even worse, the distribution of these micro-globules is very heterogeneous. It is presently believed that as DNAPLs sink through permeable soils, small quantities of free product are left behind in widely dispersed micro-globules. When the sinking DNAPL encounters a confining layer the DNAPL may accumulate and then spread laterally until it finds a fracture or some other path to the deeper zones. The heterogeneous distribution of NAPLs in the real world that has made delineating these source zones very difficult because most analytical detection schemes depend on point measurements. It is easy to see that if measurements are made at widely spaced intervals (e.g., several feet to tens of feet apart) the likelihood of locating micro globules is very remote.
At present most methods that have been used to attempt to delineate DNAPL distributions do not even attempt to target the free phase product but rather rely on extrapolation of soil gas survey results and coincidental soil and water sampling to predict the location of the source material. Current approaches that depend on use of soil gas methods for identifying free phase DNAPL below the water table are often unreliable because: 1) volatilization of DNAPLs below the water table is not always detectable within the vadose zone, 2) vapor migration pathways do not necessarily match distributions of free phase product, and 3) positive soil gas results do not provide information about the depths of the subaqueous free phase DNAPLs.
There are additional difficulties associated with the use of conventional soil and water sampling methods that depend on drilling, sampling, and laboratory analysis to locate free phase DNAPLs below the water table. Depending upon soil type, the use of conventional split spoon sampling below the water table may be questionable. Unconsolidated sands and silty soils tend to flow in the saturated zone, resulting in poor retention of the sample in the split spoon. Delineation of these subsurface contaminants usually requires trial-and-error placement of a significant number of monitoring wells and extensive sample collection efforts. Laboratory analysis of samples taken in the field is time consuming and costly. Since DNAPLs are volatile, sample-handling problems often lead to questionable analytical results. In addition, conventional sampling procedures that rely on drilling into or through DNAPL zones create an additional problem because they may actually provide new pathways for mobilization of the contaminant. Conventional monitoring techniques that use soil samples taken at selected depth levels along with permanent monitoring wells installed in clusters of two to four at different depths are believed to rarely provide the level of detail needed to provide a reliable picture of the nature and extent of solvent DNAPL below the water table. This belief is based on the understanding that because the distribution of DNAPL solvent distributions is complex, there has rarely been a direct detection of residual or free-phase DNAPL in the groundwater zone even at intensely investigated DNAPL sites.
Thus, there is a continuing need for a method for obtaining visual images of subsurface environments to detect underground chemical contamination, such as NAPLs.
The invention provides a method for detecting chemical contamination in subsurface environments. The method is implemented using a video imaging system incorporated into a probe than be pushed into soil to collect in situ images. The method is particularly useful for identifying non-aqueous phase liquids (NAPLs) contaminants. Immiscible globules of NAPLs can be detected in the in situ images based on differences in shape and/or color with respect to the soil background. Alternatively, indicator dyes that partition the NAPLs can be released from the sensor probe so that the NAPLs are rendered more easy to detect due to changes in color or a specific fluorescence emission resulting from a chemical interaction between the NAPLs and the indicator dye.
An important advantage of the invention is that it provides a method for directly observing contamination source zones in subsurface soil environments under actual conditions, as opposed to taking core samples.